The present invention relates to a circular pelletizing machine and a method of controlling the same.
More particularly, the present invention deals with a circular pelletizing machine for compressing a powder or grain material, which has a rotatable matrix disc with a plurality of circumferentially distributed matrices as well as upper and lower stamps associated with the matrix disc. Pre-pressing rollers and main-pressing rollers act on the upper and lower stamps to pre-press and to finally press the material. In this machine the main pressing rollers is adjusted by an adjusting motor which is controlled by a computer.
Circular pelletizing machines of the above mentioned general type are known in the art. They are used for manufacturing of pressed (stamped) articles of different materials. The use of such machines is not limited only to the manufacture of pellets from medical substances to be administered to patients, but also these machines are used for producing articles of synthetic plastic materials, metal powder or ceramic masses. The individual pellets are produced by compressing the powdered mass in a matrix on a rotatable matrix disc in the upper and lower stamps. The force of compression of the material in the main pressing station is obtained by displacement of the upper and lower stamps under the action of main pressing rollers, and the distance between the latter is adjusted by an adjusting motor, for obtaining the required main pressing force. The above described machine and method of its controlling is disclosed in the German patent document DT-OS No. 3,131,402.
In modern high efficiency pelletizing machines, a pre-compression of the material in a pre-pressing station is performed under the action of the pre-pressing force. This pre-pressing force is as a rule considerably lower than the main pressing force. For example, the pre-pressing force amounts to approximately 10 kN, and the main pressing force amounts to approximately 40 kN. In many cases the manufacture of pellets requires such high pressing forces that they lie very close to the permissible load limits of the stamps for pressing. In the event of operational disturbances for example double filling, the limit of the permissible load of a main pressing stamp can be exceeded, and this can lead to a crushing or a breakage of the stamp. For preventing this, it is known to provide hydraulic or spring safety devices. These devices possess however the decisive disadvantage that they are relatively slow since the masses to be moved are high. Therefore, in fast and high-efficiency machines, a sufficient displacement of the main pressing rollers within short time is not possible, since the time required for producing the signal of the main pressing force and moving the frame system of the main pressing rollers amounts to several milliseconds in the event of high pellet output. Because of the above reasons, in the pelletizing machines with high outputs a stamp breakage is not excluded, despite the safety means provided in them.